Film technique fabrication improvement for slip casting powdered materials



.. Unite mold cavity.

;' FILM TECHNIQUE FABRICATION IMPROVEMENT 1 FOR SLIP CASTING .POWDERED MATERIALS Paul E. Rempes, Jr., 4224 Oxford Drive, Columbus 21, Ohio NoDrawing. Filed May 28, 1958, Ser. No. 738,578 13 Claims. (Cl. 1175.1) (Granted under Title 35, US. Code (1952), see. 266) The invention describedherein may be manufactured and used by or'forv the United States Government for governmental purposes without paymentto me of any royalty thereon.

This invention consists of an organic film technique fabrication improvement involved in the shapingof com- .plex, thin-walled and solid components from powdered materials by the slip casting process. novel, inexpensive, easily used, reproducible and reliable.

The technique is Slip casting is a forming process in which electrolytes and deflocculants are used to maintain a controlled suspension (or slip) of fine grained powders. Numerous interrelated factors such as base exchange capacity, thixotropy, viscosity, pH, electrolytes, temperature, aging,

grain size and grain size distribution, and specific gravity To produce a component, the slip is pouredinto a porous mold and remains until a sufiicient thickness of powdered material has been deposited along the mold walls, by the carrier action of the liquid being absorbed by the mold. A solid component is formed by filling the mold until the powdered material completely fills the from the mold and sintered or fired to elevated temperatures to produce the final product.

Release of I castings from plaster (calcined gypsum.)

.- molds is considered dependent on slip interaction with the mold. Release of an acid treated pure ceramic. oxide casting from the mold results from .surface coagulation .tor flocculation due to ion exchange.

Many powdered materialsreact with acids to form chlorides, sulfates and nitrates and cannot form acid slips. Therefore to slip cast these materials the use of neutral slips was a necessity. These slips have small shrinkage and no surface .flocculation. Fine particles from the slip penetraterthe plaster pores by capillary action, creating. mechanical ,adherence of the casting to the mold.

, This invention relates. to an organic film technique which overcomes mechanical adherence by separating the casting and the mold with a thin film that permits passage through it of the liquid being absorbed by the mold, while preventing passage of even the smallest particles being cast. Release is accomplished by separation of the film from the mold and/ or casting after an appropriate drying time. The film also prevents mold and casting surface contamination, prevents mold pore clogging, 1 assists in casting time control and by use'with multiple piece molds, and permits the release'of castings having no drying shrinkage.

None of the previous techniques proposed to overcome the casting to mold sticking problem, such as paper linings,'or soapstone, graphite or olive oil applications, have had the combinations of advantages belonging to the film technique. They were either limited to simple geometric shapes, gave surface contamination or clogged the mold pores.

Basically the improvement is a thin semi-permeable A thin walled-component is formed bypouring out the excess slip after the desired wall thickness is built up. After drying the castings are removed Patented June 27, 1961 membrane type algin film which is deposited as a .uniform, coherent, continuous layer .on the mold cavity surface of a suitable porous mold, i.e., calcined gypsum or 1 plaster.

. Algins are the water soluble derivatives of alginic' acid, which is a hydrophilic colloidal polymer of anhydro-B D- mannuronic acid. Commercial derivatives of alginic acid are the sodium, potassium, ammonium andpropylene glycol alginates. A representative publication'on the subject is entitled Algins at Work published inl952 by the Kelco Company, 120 Broadway, New York 5, New York. Another publication is Ceramic Industry published January 1958 by Industrial Publications, Inc.,

5 South Wabash Avenue, Chicago 3, Illinois.

From work performed in slip casting it has been determined that each of the alginates, sodium, potassium, ammonium and propylene glycol alginates, are about 1 equally satisfactory in film forming properties and offer a considerable variation in pH values and each is usable in water solutions ranging from 0.10 wt. percent to l'.00 wt. percent alginate and accordingly the selection of the alginate to be used is primarily determined by consideration of cost, availability and pH value of the slip to-be cast. 'From general considerations sodium alginate is the preferred film forming material because of its ready availability, low cost and compatibility with slips havingpH values ranging from about 2 to 12. A standard refined sodium alginate generally available for industrial usage is produced by the Kelco Company and sold under? the trade'name Keltex. Film forming solutions range from 0.10 wt. percent to 1.00 wt. percent alginate with a corresponding viscosity range of 15 to 1,000 centipoises at 20" C. In view of the low weight percentage of alginate concentrations involved for film forming,.there is little danger of the ash residue forming undesirable surface compounds on atmospheric fired castings. .All alginate products decompose at 2l0-225 C. by carbonizing and the residual carbon can be completely burned off in air at 1250 F.

The film forming solution which is preferred contains 0.20 wt. percent sodium alginate in distilled water and has an approximate viscosity of 20 centipoises at 20 C.

- and produces films compatible with slips having pH values ranging from 2 to 12. It can be prepared by dissolving 2 gr.'of sodium alginate in 1,000 cc. of distilled water by vigorous agitation and does not require any special soaking, heating or additional treatment. The sodium alginate employed in this work was the material Keltex .sodium alginate and had a pH value which was neutral.

. ing rate and the viscosity of the solution. A two minute time intervalv issatisfactory based on use of calcined -gypsum. (CaSO -1/zH O) molds prepared by standard techniques and having a 7:10 waterzplaster ratio, an average filmthickness of .0006" (l5v microns) and use -.of the 0.20 wt. percent sodium alginate (Keltex) solution at20 C.. After 3 to 5 minutes drying, the filmlined mold wasready for use. Film thickness has ranged from .0001" to .005? depending on the type and concentrationsolution used and the film forming time. 'Ihe film is translucent, tough and elastic while moist, but it .,.shrinks noticeably when drying and thenlooks much; like tissue paper. After components are slip cast, any film v.adhereing to thecasting may be removedby stripping or may be burned from the casting, as preferred.

T0 date the film has been an effective aid in slip casting powdered materials in the following categories: carpreserved by the addition of from 0.1% to 0.2% of a preservative such as formaldehyde, one of the esters of 'p-hydroxybenzoic acid, a phenol derivative or the like. The alginateisolution pH range may be taken as being from 4 to 9 and buffering may be employed for control of the pH value.

An organic alginate film technique for slip casting powdered materials in accordance with the invention allevi ates the mold to'casting sticking problem and prevents casting and mold surface contamination. Difierent materials may be slip cast successively in the same mold without danger of contamination by use of this improvement. The film is inexpensive, conveniently fabricated, easily used, reproducible and reliable.

The invention also increases mold life by preventing mold pore clogging, assists in casting time control and by use with multiple piece molds, permits release of castings having. no drying shrinkage.

By varying the concentration and type of alginate solution used and the film forming time, the film thickness can be varied and compatibility with either acid or basic slips can be assured.

The film may be deposited on any rigid porous mold material that has a network of uniformly distributed fine open pores.

It will be understood that variations may be made in the invention which will be apparent to those skilled in the art coming within the scope of the invention as defined in the appended claims.

'I claim:

1. In the art of slip casting powdered materials the procedural steps of mixing the powdered materials with water to make a slip, pouring the slip containing the powdered material into a mold, previously providing a release coating in the mold over the casting surface by having deposited and dried thereon a film composed of a water soluble alginate as a smooth and tough water pervious and particleimpervious coat lining the slip con tacted mold cavity.

' 2. The method of providing a water pervious and particle impervious film and an improved film release for slip cast materials from porous molds consisting of coating the mold surface with a water solution of an alginate selected from the group consisting of sodium alginate, potassium alginate, ammonium alginate and propylene glycol alginate, and drying the coating to form a thin water penneable film prior to casting the slip in the mold.

3. 'I'he'method of releasing slip cast powdered materials from a porous mold wherein the slip being cast tends to plug and adhere to the surface defining the mold cavity, consisting of the steps of making a solution of an alginate selected from the group of sodium alginate, potassium alginate, ammonium alginate and propylene glycol alginate in water such that the viscosity of the solution lies within the range of from to 1000 centipoises at C.; soaking the mold cavity with the solution for a period of from one quarter of a minute to ten minutes at room temperature; and drying the mold cavity to deposit thereon a water permeable alginate film preparatory to pouring the casting slip into the mold cavity.

. 4. The practice of slip casting which includes the step of surfacing the mold cavity withan alginate film that is within the thickness range of from .0001 inch to .005 inch. and that is permeable to water and that is substantially impervious to solid particles within the slip casting range.

5. The slip casting step defined in the above claim,4 wherein the film consists of sodium alginate. 7 1' ,6.. 'I.he practice of slip casting which includes the step i 4 of surfacingthe mold cavity with a water pervious and solids substantially impervious alginate film selected from sodium alginate, potassium alginate, ammonium alginate and propylene glycol alginate.

7. In the art of slip casting powdered materials in porous molds the combination with said mold of a release film covering and bonded to the surface of the mold cavity said film being formed of an alginate layer that is pervious to water and that is impervious to solid particles;

8. The combination as claimed in claim 7, in which the film is formed from evaporation of a mold wash con sisting of an alginate selected from the group consisting of sodium, potassium, ammonium and propylene glycol alginates in the amount of from one gram to ten grams by weight dissolved in a liter of water.

9. The combination as claimed in claim 7, in which the film is formed from evaporation of a mold wash consisting of an alginate and mixtures thereof selected from the group consisting of sodium, potassium, ammonium and propylene glycol alginates, dissolved in water to give a solution having a viscosity of from 15 to 1000 centipoises at a. temperature of 20 C.

10. The combination as claimed in claim 7, in which thefilm is formed from evaporation of. a mold wash consisting of sodium alginate dissolved in water the alginate being sodium alginate dissolved in water in the amount of about two grams of the alginate to a liter of water.

11. The method of releasing slip cast powdered materials from a porous mold wherein the slip being cast is released smoothly from the surface of the mold cavity, the method steps being contacting the mold cavity with a solution ofthe composition of about two grams of sodium alginate dissolved in a liter of water, maintaining the sodium alginate-water solution in contact with the surface of the mold cavity for a period of time of about from one to five minutes, and drying the mold to leave a sodium alginate film coating over the surface of the mold cavity and the sodium alginate film having the characteristics that it is permeable to water and substantially impermeable tosolid particles for conditioning the surface of the mold cavity preparatory to the pouring of the casting slip in the moldcavity.

12. The practice of slip casting using a calcined gypsum porous mold having a mold cavity inclusive of the process steps of pouring into the mold cavity a 0.2 percent by weight water solution of sodium alginate of a viscosity of about 20 centipoises at 20 C. and having a solution pH of 7, permitting the solution of sodium alginate to wet substantially all parts of the mold cavity with a sodium alginate film having a substantially uniform thickness of about. .0006", removing from the mold cavity the sodium alginate solution not adhered to the surface of v the mold cavity after a time interval of 2 minutes, drying the mold to provide a surface film on the mold cavity that is pervions to water and that is substantially impervious to the size of solid particles used in slip casting operations, and permitting the sodium alginate film lining the'mold cavity to remain in place during the slip casting operation.

- 13. 'A film forming solution for use in applying a water pervious and solids impervious casting release improving lining to a mold cavity which solution consists of a water solution containing 0.20 Weight percent of sodium alginate and the solution having a viscosity of about 20 centipoi'ses at 20 C. for use as a film adhered to-the surface of a mold cavity for use with slips having pH values within the range of from 2 to 12.

References Cited in the file of this patent UNITED STATES PATENTS 

2. THE METHOD OF PROVIDING A WATER PERVIOUS AND PARTICLE IMPERVIOUS FILM AND AN IMPROVED FILM RELEASE FOR SLIP CAST MATERIALS FROM POROUS MOLDS CONSISTING OF COATING THE MOLD SURFACE WITH A WATER SOLUTION OF AN ALGINATE SELECTED FROM THE GROUP CONSISTING OF SODIUM ALGINATE, POTASSIUM ALGINATE, AMMONIUM ALGINATE AND PROPYLENE GLYCOL ALGINATE, AND DRYING THE COATING TO FORM A THIN WATER PERMEABLE FILM PRIOR TO CASTING THE SLIP IN THE MOLD. 